1. Technical Field
The present invention relates to a printed circuit board, a method for producing such a printed circuit board, and a layer composite material for such a printed circuit board.
2. Prior Art
In present-day electronic circuit technology concerning, in particular, computer technology and the communication and processing of data in communications technology, development is tending toward an ever greater integration and packing density and toward ever higher clock frequencies, which are normally already in the GHz range. Under these circumstances, increasing importance is being attached to the printed circuit boards or printed circuits in the construction of functionally reliable and, at the same time, cost-effective circuits. On the one hand, such a printed circuit board, in particular for relatively large series, must be simple and cost-effective to produce and process (cut, drill, populate, etc.). On the other hand, the printed circuit board must have a sufficient thermal and mechanical stability and the best possible thermal conductivity, because a considerable amount of heat is generally to be expected owing to the high integration density and high frequencies during operation. Furthermore, it is desirable for the printed circuit board to be adapted to the ubiquitously used semiconductor components (made of Si or else GaAs) with regard to thermal expansion, because this enables direct mounting of the semiconductor chips on the printed circuit board (Direct Chip Attach DCA) with all its advantages.
However, the dielectric properties of the insulation material used within the printed circuit board are particularly important with regard to the high frequencies. Thus, a printed circuit board used in the extremely high frequency range should have an insulating layer between the conductor layers with the lowest possible (relative) permittivity εr and a small dielectric loss factor tanδ in order to keep down the losses that increase with the frequency.
Finally, at the high clock rates and with very fine conductor tracks, it is becoming more and more important for the printed circuit board material to be distinguished by a high degree of homogeneity in the dielectric and a high uniformity in the external dimensions (small thickness fluctuations, etc.) because otherwise, in adjacent regions of the printed circuit board, undesirable propagation time differences are produced during the signal propagation and impair the functionality of the circuit constructed therewith.
All the requirements presented are satisfied only poorly or not at all by conventional epoxide-based substrate materials. Therefore, various proposals have already been made in the past in respect of using, for single or multilayer printed circuit boards, insulating intermediate layers made of a sintered glass ceramic which, at the same time, have good dielectric properties and are adapted to GaAs circuits, for example, in terms of their thermal expansion coefficient (see e.g. U.S. Pat. No. 6,017,642). However, such sintered glass ceramic substrates are complicated to produce and, as ceramic plates, have only a limited mechanical strength, so that they allow the realization of, in particular, thin single-layer printed circuit boards only with difficulty.
Another proposal relates to the use of a “glass paper”—produced from glass fibers—as dielectric for printed circuit boards (JP-A-9208252). Although such a fiber material should be less at risk of fracture compared with the glass ceramic, the irregular fibrous structure of the material results in a local inhomogeneity in the dielectric properties, which can lead to the abovementioned propagation time problems with high line densities and at high frequencies.
Finally, a whole class of substrate materials for printed circuit boards is known which comprise fluoropolymers filled with additives (ceramic particles, glass fibers) (see e.g. U.S. Pat. No. 5,149,590). Although such materials, which are commercially available for example from the US company Rogers Corp. under the designations RT/DUROID 5870–5880 and RO3000, exhibit relatively good and homogeneous dielectric properties and are therefore well suited to extremely high frequency applications, such a material is comparatively expensive on account of the complicated production and, moreover, has an unfavorable thermal expansion coefficient which is significantly higher than that of silicon.